Class-D amplifiers are often used in electrical circuits when high efficiency is required. For example, in battery-powered wireless communication devices utilise class-D audio amplifiers in order to provide a higher theoretical maximum efficiency of amplification of audio signals (up to 100%), as compared to the theoretical maximum efficiency of less efficient amplifier topographies, such as class AB audio amplifiers (up to 78% efficient). Class-D audio amplifiers minimize power dissipation in the output stages by working in a linear operating region only (e.g. the well-known transconductance region). This is illustrated in FIG. 1, which shows a graph 100 of a drain-source voltage 105 versus drain-source current 110 for a class-D amplifier, indicating the transconductance region 120.
One advantage in employing class-D amplifiers is the reduction in the required surface area of silicon. This is due to the fact that a power metal oxide semiconductor (MOS) device is sized with respect to the intrinsic resistance (Ron) associated with the power stage of the class-D transistors when operating in a switch region 115. This is compared to the size of a MOS device being dependent on the saturation voltage in a class AB device.
Referring now to FIG. 2, a classic class-D amplifier (analogue) topology is illustrated. The topology is illustrated within a classic pulse wave modulator (PWM) 225 design. As known, the PWM 225 converts an analogue input 210 having a given bandwidth 215 into a 1-bit digital stream 230. In terms of spectrum usage, this digital stream 230 therefore includes an analogue input spectrum and its alias, repeated at the sampling frequency 255. This digital stream 230 is configured to drive a bridged power stage 235, which delivers power to the speaker 240.
In this manner, a classic class-D amplifier topology offers the advantage of greater efficiency and reduced power dissipation over class AB power amplifiers. Furthermore, and notably, class-D amplifiers do not require the use of a specific common mode reference.
However, this known prior art has the disadvantage that, in general, class-D audio power amplifiers are renowned as having a poor behavior if there are perturbations in the power source, for example when a battery is used. This poor behavior is accountable to a large degree by inter-modulated frequency components between the supply and the audio signal. Consequently, class-D amplifiers do not lend themselves readily to use in wireless communication devices, where a reliable and consistent power source cannot be guaranteed.
Furthermore, it is also known that the power gain provided by class-D power amplifiers is dependent upon the power supply. Such a dependency, requiring a consistent and stable power supply, is undesirable. Again, this is particularly the case in portable (battery-powered) devices, such as mobile phones, where the battery voltage continuously varies over the life of the battery.
The aforementioned problems with analogue designs are also applicable to class D audio power amplifiers that incorporate a digital PWM design. Furthermore, with a digital design, over-sampling of the digital stream is required, which adds yet further complexity to the design.
Thus, a mechanism to improve the performance of an audio amplifier is needed, particularly one with high efficiency and immunity to supply variations. In particular, a mechanism to reduce the performance dependency of class-D amplifiers on power supply, particularly in the context of audio power amplifiers in a wireless communication device, would be advantageous.
A need therefore exists for an improved arrangement wherein the abovementioned disadvantages may be alleviated.